Two New Late Triassic Phytosaur Papers

Stocker, M. R.. 2013. A new taxonomic arrangement for Paleorhinus scurriensis. Earth and Environmental Science Transactions of the Royal Society of Edinburgh (advance online publication)DOI: http://dx.doi.org/10.1017/S1755691013000340

Abstract - The paraphyletic genus ‘Paleorhinus’ is understood currently as a cosmopolitan phytosaur taxon from the Late Triassic. There is no consensus regarding the number of species of ‘Paleorhinus,’ with multiple species and genera synonymised into a single genus or even a single species at various points in its published history. The taxonomy is confounded by historical descriptions without the benefit of comparisons to more recently collected specimens, emphasis on plesiomorphic cranial morphology as diagnostic features of the genus,and lack of cladistic analyses. When included in a recent explicitly cladistic phylogenetic analysis, the holotype of ‘Paleorhinus’ scurriensis (TTU P-00539) was found to be the earliest-branching phytosaur with respect to other North American specimens previously referred to ‘Paleorhinus,’ and is generically distinct from Paleorhinus. ‘Paleorhinus’ scurriensis differs from all known phytosaurs in five unambiguous characters: basitubera widely separated mediolaterally; ridge present on lateral surface of jugal; thickened shelf present along posteroventral edge of expanded pterygoid-quadrate wing; ‘septomaxillae’ separated and excluded from internarial septum; and nasal swelling present posterior to posterior borders of nares. This detailed morphological description of an early-branching phytosaur taxon is a first step towards resolving long-standing issues surrounding specific anatomical features and relationships among early members of the clade.Hungerbühler, A., Mueller, B., Chatterjee, S., and D. P. Cunningham. 2013. Cranial anatomy of the Late Triassic phytosaur Machaeroprosopus, with the description of a new species from West Texas. Earth and Environmental Science Transactions of the Royal Society of Edinburgh (advance online publication) DOI: http://dx.doi.org/10.1017/S1755691013000364

Abstract - The skull anatomy of a new species of the phytosaur Machaeroprosopusis described for the first time on the basis of two specimens from the Upper Triassic Cooper Canyon Formation of Texas. Additional information is provided by a third specimen referred to Machaeroprosopus sp. A paranasal bone, an additional paired element of the narial region, is identified. Important new data are presented for the braincase, including the morphology of the epipterygoid and presphenoid, an anterior process of the prootic, an anteroventral process of the laterosphenoid, and a parasphenoid process. Machaeroprosopus lottorum n. sp. is characterised by four apomorphies: a supratemporal fenestra closed on the skull roof with beveled anterior rim, a comparatively short squamosal, a flat and rugose narial rim, and medially extended palatines that come close to form an ossified secondary palate. With respect to the supratemporal fenestra, the supraoccipital–parietal complex and several features of the squamosal, Machaeroprosopus lottorum n. sp. bridges the morphological gap between species previously referred to the genera Pseudopalatusand Redondasaurus. A parsimony analysis of known species of Machaeroprosopus supports the hypothesis that the development of the rostral crest in Machaeroprosopus is a sexually dimorphic feature, and questions the validity of the genus Redondasaurus. Consequently, Redondasaurus is here considered a junior synonym of Machaeroprosopus.

7 comments:

While browsing Wikipedia, I found out that Stocker has coined the new genus Wannia for "Paleorhinus" scurriensis, apparently to honor Wann Langston Jr for his contributions to the study of fossil vertebrates in Texas and Oklahoma. No surprise, given that the paraphyly of Paleorhinus has been hinted at before and confirmed by the cladistic analysis of Stocker (2010).

I was wondering, are there studies in prep/press about re-describing the species Machaeroprosopus andersoni, M. buceros, M. pristinus and M. tenuis? Long and Murry (1995) referred many specimens to some of these, but it seems like nobody cares... And I would expect to see some stratigraphical sense in these referrals, I mean, different species overlapping like that...

A redescription of many of the Machaeroprosopus type specimens is sorely needed to fully evaluate the taxonomic status of these specimens so that we can refer other material. However, this is a PhD scale project. Anyone looking for a PhD project?

If there is any question about the current identification of phytosaur specimens referred to M. buceros and M. pristinus by Long and Murry, it's important to note that:

(1) the pseudopalatine skulls from the Bull Canyon Formation of New Mexico that were referred to M. buceros by Long and Murry were re-identified as M. mccauleyi by Hunt et. al. (2006)

(2) Long and Murry didn't consider Redondasaurus distinct from Machaeroprosopus, so they considered all phytosaur skulls that were assigned to Redondasaurus to be M. buceros and M. pristinus, including the skull from Oklahoma that Hunt and Lucas referred to Redondasaurus gregorii.

(3) the referral of all phytosaur skulls from the Owl Rock Member to M. buceros by Spielmann et. al. (2007) was done on the assumption that M. buceros and M. pristinus are sexual dimorphs of the same species, something that is not supported by available evidence

(4) Long and Murry considered M. andersoni synonymous with M. buceros without justification, and they also tentatively suggested that M. validus might also be the same animal as M. buceros

Hunt, Adrian P.; Lucas, Spencer G.; Spielmann, Justin A., 2006: Sexual dimorphism in a large brachyrostral phytosaur Archosauria Crurotarsi from the Late Triassic of western North Africa. Bulletin of the New Mexico Museum of Natural History and Science 37: 563-567

I agree that the whole thing is a mess and needs to get readdressed using an apomorphic approach that assigns specimens to clades based on discrete characters and minimizes stratigraphic and regional bias. A similar approach is being done by Michelle Stocker for the non-phytosaurid phytosaurs. All of the Long and Murry (1995) assignments should be taken with a grain of salt because they did not use this type of approach. The same goes with any of the NMMNS Bulletin papers which do not use an apomorphy based approach, use plesiomorphies to refer taxa, and are not adequately peer-reviewed. There are also errors such as the Rock Point (Wingate) phytosaur assigned to the Owl Rock in the Speilmann et. al paper. Please be wary of any information in these references.

The sexual dimorphism hypothesis, first put forward by Colbert (1947) and then reiterated by Zeigler et al. (2002, 2003) is gaining support. See the recent Hungerbuhler et al paper. BTW...the two Zeigler et al references are the exact same paper published in two different places. You will often see them cited as separate (as I did above) but only one should be.

See Irmis (2005) for a discussion of why the sexual dimorphism hypothesis remains untested (and the fact that there are other potential explanations that explain available data equally well). Also, if M. pristinus and M. buceros are sexually dimorphic, why haven't any M. buceros specimens been found in the Petrified Forest area, where we have a number of specimens referred to M. pristinus?

Actually the phytosaur skull we collected from the cliff above Rainbow Forest has coded out as M. buceros using the Hungerbuhler et al matrix. I'm working on a description of that skull but we really need a full redescription of the type skull of M. buceros.

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My name is Bill Parker and I am a paleontologist in northeastern Arizona. While my day job mainly involves roaming the badlands of Petrified Forest National Park picking up bit after bit of phytosaur scrap (and finding some good stuff in-between), thus my main focus is the paleontology of the Triassic Period. However, I also have a very strong interest in Civil War history, and am a direct descendent of veterans who fought on both sides. Note: This is a personal site and all posts are my own opinions and do not represent the opinions of the National Park Service.